The present application claims priority to Japanese Application No. P2000-067345 filed Mar. 10, 2000, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to a method of mounting chips, and particularly to a method of mounting a large number of fine chips, for example, fine semiconductor chips having sizes of the order of m on boards.
Functional devices formed of semiconductor chips such as LEDs and ICs having fine structures have been mounted on boards by forming the semiconductor chips into shapes such as trapezoidal shapes by etching while providing recesses corresponding thereto on the board side, wherein the semiconductor chips are mounted in the recesses.
The above method, however, has the following problem: namely, in the case of using semiconductor chips made from materials not easy to be formed into trapezoidal shapes by etching or of fine dimensions, the mounting of these semiconductor chips becomes difficult, or the accuracy in shape of the chips and/or the positional accuracy in mounting of the chips are lowered, thereby degrading the mounting yield.
In mounting semiconductor chips such as light emitting devices used for a display on a board, the semiconductor chips have been required to be mounted on a board surface having a specific area with a high efficiency and a high yield, and further, in consideration of the cost. Further, along with the tendency toward miniaturization of semiconductor chips, the handling of the semiconductor chips at the time of mounting have come to be difficult. Accordingly, it has been expected to develop the semiconductor chip structures capable of effectively coping with the above terms, that is, the efficiency, yield, cost, and handling at the time of mounting, and to develop the method of desirably mounting these semiconductor chip structures on boards.
One method of mounting fine chips such as fine semiconductor chips has been disclosed, for example, in U.S. Pat. Nos. 5,545,291, 5,824,186, and 5,904,545, Japanese Patent Laid-open (Translation of PCT International Publication) No. Hei 9-506742, and Japanese Patent Laid-open No. Hei 9-120943. In this mounting method, each chip is tapered to be determined in its vertical direction, and buried in the corresponding recess formed on a board. In this case, the chips are mixed in a solution such as water or alcohol into a slurry, and the slurry is made to flow on the board.
Another method of mounting fine chips, such as IC chips, using static electricity for mounting the IC chips on a board has been described in U.S. patent application Ser. No. 07/902986. In this mounting method, the chips are vibrated by static electricity and are arranged in a state in which the positional energy of the chips is minimized.
According to the former mounting method characterized by tapering semiconductor chips, although a sufficiently large yield can be kept for semiconductor chips whose sizes are in the range of several hundreds xcexcm or more; however, the yield is lowered for semiconductor chips whose sizes are in the range of several tens xcexcm or less. Accordingly, this method has a problem from the practical utilization.
The latter mounting method characterized by using static electricity requires an apparatus for vibrating fine chips with static electricity. Further, according to this method, since fine chips collide with each other by mechanical vibration, there may occur an inconvenience that part of each chip be damaged. From this viewpoint, this method is unsuitable to practical use.
An object of the present invention is to provide a method of mounting chips, typically, fine semiconductor chips, which is capable of mounting the chips at desired positions on a board with a high efficiency and a high yield.
To achieve the above object, according to the present invention, there is provided a method of mounting chips, including the steps of: dipping a mounting board in a solvent in which chips are dispersed; and mounting the chips on the mounting board by forming electric field gradients on the mounting board in correspondence with positions where the chips are to be mounted.
With this configuration, since fine chips such as fine semiconductor chips are mounted on a mounting board not by physical forces based on the shapes of the chips but by electric field gradients, the fine chips can be positively mounted on the mounting board. Further, since fine chips dispersed in a solvent are mounted on a mounting board by forming electric field gradients, a large number of the same kind of the chips can be simultaneously mounted by one process, to thereby increase the process efficiency and enhance the productivity.
The above-described mounting method of the present invention preferably includes the step of applying a surface treatment agent for controlling surface energy on the chips and/or the mounting board in correspondence with positions where the chips are to be mounted.
With this configuration, since the surface energies of a semiconductor chip and a mounting position of the mounting board are made hydrophobic or hydrophilic, it is possible to enhance the absorption ability of the chip to the mounting position, and hence to more positively mount each chip at a desired position on the mounting board.
The above-described mounting method of the present invention preferably the steps of providing a ferromagnetic body or a ferrimagnetic body at a specific position of each of the chips, and specifying the mounting direction of each chip by applying a magnetic field to the ferromagnetic body or the ferrimagnetic body.
With this configuration, since a portion, located at a specific position, of each of fine chips such as fine semiconductor chips is formed of a ferromagnetic body or a ferrimagnetic body, a magnetic field is formed between the above portion of the chip and a mounting board to produce a magnetic flux therebetween, so that the mounting direction of the chip can be controlled by a magnetic attraction force caused by the magnetic flux. As a result, it is possible to positively mount semiconductor chips with their postures arranged in a specific direction, and also to enhance the absorption ability of each chip by the magnetic attraction force and hence to improve the reliability in mounting.
In the above-described mounting method of the present invention, preferably, the solvent is circulated.
With this configuration, since a large number of the chips dispersed in the solvent sequentially come close to the mounting board with a specific probability by circulation and are attracted thereto by the electric field gradients and magnetic forces, it is possible to positively mount a large number of chips on the mounting board with a high efficiency.
The function of the present invention will be described below.
The present invention is intended to provide a method of mounting devices having fine structures on a mounting board and a structure obtained by the mounting method. In particular, the mounting method of the present invention is characterized in that the devices having fine structures are carried via a fluid on the upper surface of a mounting board having portions at which the devices are to be joined or received. The device receiving portion is exemplified by a recess; however, it is not limited thereto. During carrying of the devices having fine structures in the fluid, forces due to an electric field or the like are applied to the devices, with a result that the devices are mounted in the device receiving portions such as recesses in self-alignment. The structure obtained by mounting the devices on the mounting board is represented by a display in which light emitting diodes (LEDs) are mounted on a board having wiring; a signal processor; or a structure in which a semiconductor device is mounted on another semiconductor device or another plastic board. Specific examples of the kinds of devices having fine structures or fine structures (fine chips) may include a laser, a transistor, a gun oscillator, an integrated circuit, a solar connector, or a fine phosphor particle.
According to the present invention, fine chips each having a size of the order of xcexcm are mounted on a mounting board having chip receiving portions and other regions on the upper surface. The chip receiving portion is exemplified by a recess; however, it is not limited thereto. The mounting board may be made from silicon, gallium-arsenic, glass, a ceramic material, or a plastic material. The mounting board may have one or more chip receiving regions formed by forming wiring on the upper surface thereof as described above. The mounting board may be a plastic sheet.
The mounting process includes a step of preparing chips having fine structures; a step of carrying the fine structures (chips) in a fluid to form a mixture, typically, a slurry; and a step of optimally controlling the flow rate of the slurry and the external forces applied to the chips so that the chips are disposed in a region having at least one chip receiving portion, to mount each chip in the chip receiving portion in self-alignment.
According to the present invention, there is provided an apparatus for mounting fine structures on a mounting board having at least one or more receiving regions and other regions via a fluid. The apparatus may include a power source (voltage supply source) for controlling an electric field for mounting the fine structures, and a magnetic field generator for controlling the mounting directions of the fine structures.
The mounting method and the structure obtained by the mounting method according to the present invention are typically applied to a method of mounting GaAlP based LED chips on a glass board, and the structure obtained by the mounting method. The chip may be formed into any shape which be symmetric or asymmetric, for example, a cylindrical shape, a pyramid shape, a rectangular paralleopiped shape, a cubic shape, a T-shape, a granular shape, and a combined shape thereof. In general, the chip can be tightly inserted in a desired region on the mounting board depending on the shape of the chip. The chip having a fine structure may be formed of one or more layers made from GaAlAs, gallium-arsenic, silicon, diamond, germanium, and a GaN based compound, and further, other III-V compounds and II-VI compounds. The multi-layer structure of the chip can be typically formed by layers of a metal, an insulator such as silicon oxide or silicon nitride, and a combination thereof.
The chips having fine structures are mixed in a solvent to form a slurry-like mixed solution. The mixed solution is circulated by a circulating device in order that the chips are not sunk. A board for assembly (mounting board) on which the chips are to be mounted and an auxiliary electrode plate (counter electrode plate) are dipped in the circulating solution and are connected to a voltage supply source. In addition, depending on the kind of solvent, the chips can be prevented from being sunk due to a difference in specific gravity between the solvent and the chips. In such a case, it is not required to circular the solvent.
The fine chips or particles dispersed in the slurry-like mixed solution can be accurately mounted at desired positions on the mounting board by making use of forces caused by electric field gradients. In this case, electrodes may be formed on the mounting board into a pattern allowing suitable selection of the electrode positions, for example, into a matrix pattern. In this case, by controlling a voltage applied to each electrode position so as to adjust the electric field at the electrode position, each fine chip can be selectively attracted to the electrode position.
The probability in absorption of each chip can be enhanced by positively changing the surface energy of the chip by providing a layer of a surface treatment agent on the chip. In this case, not only the chip side but also the mounting board side may be selectively coated with the surface treatment agent. By combining the effect of providing the layer of the surface treatment agent with the effect of imparting the electric field gradient, it is possible to more positively attract each chip at a desired position.
For chips each having a symmetric shape or no directivity, the mounting directions thereof are not required. On the contrary, chips having an asymmetric shape or a directivity are required to be controlled in mounting directions thereof. In this case, a specific portion of each chip may be formed of a ferromagnetic body or a ferrimagnetic body, and an external magnetic field be applied to the chip or a magnetic field be applied thereto from an electrode formed on the mounting body, to generate a magnetic force, thereby controlling the mounting direction of the chip.
Each of the electrode and the magnetic body may be made from any metal insofar as such a metal does not exert adverse effect on the subsequent steps, for example, it has a property being less corroded with the solvent, not affected by electrolysis, and good in compatibility with the surface treatment agent.